Container for storing a liquid, and use thereof

ABSTRACT

The invention relates to a container for storing a liquid, which tends to decompose into gaseous decomposition components in the case of the conditions prevailing in the container ( 1 ) and in the case of which a chemical reaction equilibrium results between gaseous decomposition components and liquid, wherein a floating roof ( 29 ) is accommodated in the container ( 1 ) and the floating roof ( 29 ) comprises floats ( 33 ), using which the floating roof ( 29 ) floats on the liquid, and wherein the floating roof ( 29 ) is guided using a sliding seal ( 45 ) in the container ( 1 ). 
     The invention furthermore relates to a device for storing heat, comprising a first container ( 57 ) for storing a colder liquid and a second container ( 59 ) for storing a hotter liquid and a use of the container and the device for storing heat.

The invention is directed to a container for storing a liquid, which tends to decompose into gaseous decomposition components in the case of the conditions prevailing in the container, and in the case of which a chemical reaction equilibrium results between gaseous decomposition components and liquid. The invention furthermore relates to a device for storing heat, in which such a container is used, and a use of the container or the device for storing heat.

Liquids which tend to decompose into gaseous decomposition components are, for example, molten salts, which are used as a heat carrier medium and heat storage medium. Molten salts are used in particular where classic heat carrier media and heat storage media can no longer be reasonably used as a result of the required high temperatures. An important field of use of molten salts as a heat carrier medium are solar power plants, in which the heat carrier medium is heated in receivers by solar radiation and temporarily stored in a hot store. Water is vaporized and superheated using the hot heat carrier medium and a generator is driven to generate power using the superheated steam.

In particular in the case of molten salts, which are used in solar power plants, for example, parabolic trough solar power plants, Fresnel solar power plants or tower solar power plants, and which are based on nitrates or nitrites of the alkali metals or alkaline earth metals, wherein a mixture of nitrates and nitrites is frequently used, the risk exists that the salt will decompose to form gases as a result of the high temperatures. Thus, for example, nitrate salts of alkali metals and alkaline earth metals form the respective corresponding alkali metal oxides or alkaline earth metal oxides, respectively, at high temperatures while simultaneously forming nitrogen monoxide and nitrogen dioxide, summarized hereafter under the term nitrogen oxides. The nitrogen oxides physically dissolve in the molten salt and can back-react with dissolved alkali metal oxides or alkaline earth metal oxides in the meaning of a chemical reaction equilibrium. The nitrogen oxides can also pass into the gas state in particular in the event of sinking pressure or increasing concentration, however, and are then no longer available for a back reaction. In this way, a harmful accumulation of alkali metal oxides or alkaline earth metal oxides can occur in the molten salt.

Since the decomposition of the nitrate salts is an equilibrium reaction, the nitrogen oxides dissolved in the molten salt also inhibit the further decomposition of the nitrate salt. This safeguard becomes less effective due to outgassing of the nitrogen oxides and the reduction linked thereto of the concentration of nitrogen oxides in the molten salt, and the salts in the molten salt can decompose further.

The formation of the oxides due to the decomposition of the nitrate salts is disadvantageous. On the one hand, the decomposition reaction results, in the case of melts having a high nitrate content, in sinking of the nitrate concentration and thus a rise of the melting point. On the other hand, the corrosivity of the melt increases in relation to the metallic materials which are typically used, in particular steel. Furthermore, solids can form in the molten salt because the solubility limit of the alkali metal and alkaline earth metal concentration is exceeded, and these salts can result in abrasion on the surfaces of the facility parts through which flow occurs and therefore also in damage to the facility parts. In addition to the abrasion by entrained solid particles, it is also possible that solids will precipitate from the molten salt and result in deposits and baked-on material on the facility parts. This can furthermore result in the blocking of pipelines or heat exchangers.

It is presently typical, for example, to regenerate the molten salt as described in WO-A 2014/114508, to lengthen the service life of molten salts containing nitrates.

Alternatively, the possibility also exists of covering the molten salt with a gas phase, the content of nitrogen oxides of which is sufficiently high that a sufficiently high concentration of dissolved nitrogen oxide is obtained in the molten salt, and the decomposition of the nitrate salts can thus be inhibited. In particular in the case of use in large containers, which are used, for example, as heat stores in a solar power plant, however, this has the disadvantage that the heat stores are subjected to cyclic heating and cooling as a result of the cyclic operation, which result in substantial pressure and volume changes in the gas compartment in particular. Because of the large volume changes, it is difficult to transfer out a sufficiently large quantity of nitrogen oxides and provide them again for regeneration. On-location production would therefore be necessary for providing a sufficiently large quantity of nitrogen oxides.

Keeping the gas compartment in a state which manages without relevant emission of gases to the environment by way of consistent sealing in a gas pendulum system together with the use of gas pressure stores or gas volume stores is known. In this way, it is not necessary to supply large quantities of nitrogen oxides or starting products for the production of nitrogen oxides. However, it is disadvantageous that an additional large investment expenditure and maintenance expenditure are linked to the required use of the gas pressure stores or gas volume stores. The container is terminated on top toward the environment using the floating roof in the case of the known floating roof tanks.

For liquids which have a high vapour pressure, for example, in petrochemistry, using floating roof tanks in which a roof floats in a movable manner on the liquid in the container is known. The roof can be sealed by membranes or friction systems. Such floating roof tanks are known, for example, from U.S. Pat. No. 2,536,019 or U.S. Pat. No. 4,371,090. Furthermore, JP-A 56484887 describes a floating roof which is used in a hot water tank. However, none of the tanks described here is used under the conditions prevailing in a solar power plant, in particular at the prevailing temperatures of the heat carrier medium in a solar power plant.

The object of the present invention was to provide a container for storing a liquid, in particular a heat carrier medium in a solar power plant, which tends toward decomposition into gaseous decomposition components in the case of the conditions prevailing in the container and in the case of which an equilibrium results between gaseous decomposition components and liquid, which does not have the disadvantages known from the prior art.

The object is achieved by a container for storing a liquid, which tends toward decomposition into gaseous decomposition components in the case of the conditions prevailing in the container and in the case of which a chemical reaction equilibrium results between gaseous decomposition components and liquid, wherein a floating roof is accommodated in the container and the floating roof comprises floats, using which the floating roof floats on the liquid, and wherein the floating roof is guided using a sliding seal in the container.

In contrast to the known systems, in which the gas which results due to the decomposition of the liquid is stored in a central gas store, the size of the gas store can be greatly reduced or a gas store can even be dispensed with by way of the floating roof. The gas collects in a gas compartment underneath the floating roof and exit of the gas into the environment or into a gas phase in the container above the floating roof is prevented. In this way, damage to the liquid, in particular a molten salt containing nitrates, can be prevented or at least greatly slowed.

A further advantage results in the case of use in a two-store system, in which hotter liquid is stored in a first container and colder liquid is stored in a second container, wherein the first and the second container are connected to one another, so that liquid can be removed from the first container, cooled, and introduced into the second container or alternatively liquid can be taken from the second container, heated, and introduced into the first container. Thus, for example, in a solar power plant, the liquid from the second container is heated by incident solar radiation either in a solar field of a parabolic trough or Fresnel solar power plant or in a central receiver of a tower power plant and introduced into the first container. The liquid from the first container is used to vaporize and superheat water, wherein heat is emitted. The liquid thus cooled is then introduced into the second container. Since the liquid level in the first container and in the second container cyclically changes due to the operation, the gas volume above the liquid in the container also changes. Typically, the gas is transferred in each case from the container into which the liquid is introduced via a gas pendulum system into the container from which the liquid is removed. A molten salt is suitable in particular as a liquid which is used in a solar power plant as a heat carrier medium. Typical salts, which are used in the form of their melts, are nitrates or nitrites of the alkali metals and the alkaline earth metals and also arbitrary mixtures thereof. A mixture made of potassium nitrate and potassium nitrate is particularly preferred in this case.

However, the hotter liquid and the colder liquid have large temperature differences in a solar power plant. This has the result that the gas in the first container having the liquid having higher temperature has a very much greater specific volume at equal pressure than the gas in the second container having the colder liquid. To prevent the pressure from rising in the first container as a result of the greater specific volume of the gas, it is necessary to remove gas from the system or temporarily store it in a gas store during filling of the first container and emptying of the second container.

If the container according to the invention having floating roof is used in such a system as the first container for the storage of the hot liquid, the floating roof is preferably embodied so that the floating roof has at least one chamber, which contains thermally insulating material. Thermal insulation of the liquid in relation to the gas compartment formed above the floating roof is thus achieved. The insulation of the floating roof is preferably designed in this case so that the gas in the gas compartment of the first container has the same temperature as the gas in the second container. In this way, pressure variations of the gas can be equalized as a result of the same specific volume at equal temperature and equal pressure. It is therefore no longer necessary to additionally provide a gas store, in which the gas can be temporarily stored.

In such a system having two containers, it is also possible to provide a floating roof in the second container having the colder liquid. The floating roof has the task here in particular, however, of preventing foreign materials, for example, carbon dioxide, water or aerosol particles, in particular chlorinated aerosol particles, from being able to reach the liquid from the gas phase.

The penetration of gaseous contaminants from the gas phase above the floating roof into the liquid or of decomposition gases formed from the liquid into the gas phase above the floating roof is prevented by the use of the gas-tight sliding seal. In particular if the container is used as a heat store in a solar power plant, seals made of organic materials, in particular made of polymers such as polytetrafluoroethylene, cannot be used due to the high temperatures of the liquid accommodated in the container, specifically the molten salt used as the heat carrier medium. One possibility is to provide membrane seals which are manufactured from stainless steel. In this case, the membrane seals have at least one membrane, which presses in a springy manner against the inner wall of the container. In the case of large containers, as are used as heat stores in solar power plants, it is also possible to embody the membrane seal without contact to the inner wall of the container. In this case, complete sealing is not achieved, but the emission of nitrogen oxides from the molten salt, which contains nitrate salts, used as the heat carrier is sufficiently reduced in this way so that a sufficiently long service life of the molten salt is achieved. However, a sliding seal having membranes which press in a flexible manner against the wall of the container to obtain a gas-tight seal is preferred.

To obtain complete sealing against exiting or entering gases, it is furthermore advantageous if the sliding seal is thermally insulated against the liquid stored in the container. In this case, the sliding seal may be arranged in a region of the container having lower temperature, so that more temperature-sensitive materials can also be used as the seal material. A further advantage of the thermal insulation and the arrangement in a region having lower temperature is that the sliding seal is subjected to less corrosion, since the corrosivity increases with rising temperature in particular in the case of molten salts. Since the sliding seal is to prevent the exit of gases from the liquid or the entry of contaminants into the liquid, contact of the sliding seal with the liquid is also not required.

Further improved sealing can be achieved in that sealing chambers are arranged below the sliding seal on the floating roof. The sealing chambers can comprise multiple membranes, as can the sliding seal also, for example, these membranes pressing against the inner wall of the container, wherein there is a sufficiently large interval in each case between the individual membranes so that the membranes do not touch even during movement of the floating roof.

The thermal insulation of the sliding seal can be implemented, for example, in that insulation is applied around the circumference of the floating roof between the liquid and the sliding seal. Such insulation can be implemented, for example, by multiple parallel ring-shaped ribs along the circumference of the floating roof. Gas cushions which have an insulating effect form between the ring-shaped ribs. Alternatively, it is also possible to introduce an insulating material, for example, inorganic fibres having a high proportion of Al₂O₃, i.e., having a proportion of Al₂O₃ of at least 80%, between the ribs. If the insulation is embodied by multiple parallel ring-shaped ribs along the circumference of the floating roof and a gas cushion between the ribs, the insulation can simultaneously also assume the function of the sealing chambers. If an insulating material is used, it is particularly preferably provided with a steel jacket because of the corrosivity of the molten salt.

To prevent the sliding seal and possibly the membranes of the seal chambers or the ribs of the insulation from exerting an excessively large force on the container wall when the floating roof is moved, it is preferable if the floating roof is constructed from at least two segments, wherein the segments are movably connected to one another. The force action on the inner wall of the container can result, for example, in that the walls do not extend ideally with continuous constant spacing, but rather deviate from the ideal profile due to manufacturing tolerances. By way of the movable segments, the floating roof can be moved up or down inside the container in the event of rising or falling liquid level, respectively, without tilting or jamming.

To enable interference-free movement of the floating roof and to hold the floating roof at its position inside the container, it is preferable if the floating roof is guided on at least one guide in the container. The guide can be formed, for example, in the form of a rail on the container inner wall and a groove, which runs on the rail, on the floating roof. Alternatively, it is also possible, for example, to provide guide rods in the container interior and to form openings in the floating roof, through which the guide rods are guided.

In one embodiment of the invention, feedthroughs are formed in the floating roof. Installations can be guided through the feedthroughs through the floating roof into the liquid. Thus, for example, a submersible pump can be provided, using which the liquid can be pumped out of the container. The pump shaft for operating the submersible pump, which is typically guided in a pump shaft guide, and a flow pipe for removing the liquid can be guided in an envelope pipe in this case, for example, wherein the envelope pipe is guided through the feedthrough in the floating roof. The envelope pipe is particularly advantageous if the pump shaft, the pump shaft guide, and the flow pipe are embodied as segmented, as is typical in particular in the case of long submersible pumps. This prevents liquid from the container from being able to penetrate into the pump shaft and damage it in the region of the connecting points of the individual segments, for example. If the pump shaft guide and flow pipe are not segmented, the envelope pipe can also be emitted. In this case, pump shaft guide and flow pipe are each guided through separate feedthroughs in the floating roof.

Furthermore, a dip tube, through which the liquid is introduced into the container by a bottom-up filling, can also be guided as an installation through a feedthrough in the floating roof. To damp oscillations during the introduction of liquids, it is possible to fix the dip tube on the container floor. For this purpose, for example, the dip tube can be inserted into a liquid distributor and clamped therein.

A further possibility for damping oscillations is to provide a baffle plate below the mouth of the dip tube in the container. During the introduction of liquid, it firstly flows against the baffle plate and is deflected in this case. Influence can be taken on the flow within the liquid by suitable geometry of the baffle plate. The baffle plate can be provided with openings or can be formed as conical, for example.

To prevent gas from being able to flow out of the gas compartment above the liquid into the gas compartment above the floating roof or contaminants or gases from being able to reach the liquid from the gas compartment above the floating roof at the feedthroughs in the floating roof, the feedthroughs are preferably sealed using a suitable seal. For this purpose, it is possible, for example, to seal the feedthroughs using a movable sealing plate. It is ensured by the movable sealing plate that the sealing plates do not exert excessively large forces on the installations when the floating roof rises or falls. For this purpose, the movable sealing plates are designed so that they can move horizontally on the floating roof. At the same time, the sealing plates must be fastened on the floating roof so that they can move with it during the rising and falling of the floating roof and do not remain hanging at one position. The sealing plates preferably lie loosely on a level surface on the upper side of the floating roof and are guided by the installations. Therefore, small manufacturing and mounting deviations of the installations from the ideal perpendicular profile can be compensated for. Alternatively, it is also possible to implement the sealing of the feedthroughs using elastic sliding seals.

In particular if the liquid in the container is a molten salt, which tends to creep, it is advantageous if the sliding seal has protective units against liquid which creeps upward. This prevents the sliding seal from coming into contact with the liquid and being damaged by the liquid, for example, by corrosion. A drip edge can be formed on the floating roof as a protective unit against liquid creeping upward, for example. In addition, a minimum spacing between surface of the liquid and sliding seal is to be maintained. The minimum spacing is preferably at least 50 cm in this case.

During usage of the container as a heat store in a solar power plant, high temperature differences can occur between the lower side of the floating roof and the upper side of the floating roof. These result due to the high temperature of the liquid, generally of 450 to 550° C., and the colder gas in the gas compartment above the floating roof. This is the case in particular if the floating roof is embodied as thermally insulating. To compensate for the differing thermal expansion occurring as a result of the temperature differences and the stresses linked thereto, the floating roof preferably has units to compensate for thermal expansions. Since the temperature differences remain substantially constant in normal operation, for example, compensation sections and/or a suitable pre-tension can be provided as units to compensate for thermal expansions.

In order that the floating roof floats on the liquid, it is equipped with floats. In order that the floating roof is not immersed in the liquid, even over a long operating time, it is necessary for the floats to maintain their volume and not be compacted. This could occur, for example, due to high pressure or pressure variations. For a pressure-resistant embodiment, for example, it is possible to fill the floats with an insulation material having low density and high pressure resistance. Suitable insulation materials of this type are, for example, ceramics having gas inclusions, for example, ceramic foams.

The container is particularly preferably used in a solar power plant as a heat store. However, usage is also conceivable in any other arbitrary device in which a liquid is used which tends under storage conditions to decompose while forming gaseous decomposition products, wherein the liquid and the gaseous decomposition products are in chemical reaction equilibrium.

A device for storing heat comprises a first container for storing a colder liquid and a second container for storing a hotter liquid, wherein the containers are connected to one another so that the colder liquid flows out of the first container, after absorbing heat, into the second container and flows out of the second container, after emitting heat, into the first container, wherein at least the second container is a container as described above.

Such a device for storing heat is particularly advantageously used in solar-thermal power plants, solar power plants in short, for example, parabolic trough, Fresnel or tower power plants.

In a particularly preferred invention, a gas compartment is formed in each case in the first container and in the second container above the floating roof and the gas compartments of the first and the second container are connected to one another via a connecting line. The gas can flow in each case from the container which is filled into the container which is emptied through the connecting line. A pressure equalization is implemented in the respective containers in this way without additional gas supply.

Exemplary embodiments of the invention are illustrated in the figures and will be explained in greater detail in the following description.

In the figures:

FIG. 1 shows a container having a floating roof according to the invention,

FIG. 2 shows a detail of the floating roof,

FIG. 3 shows a schematic illustration of a solar-thermal power plant in which a container having floating roof is used.

FIG. 1 shows a container having floating roof designed according to the invention.

A container 1, as is used, for example, in a solar-thermal power plant as a store for hot heat carrier medium, in particular a molten salt, comprises a container floor 3, a container wall 5, and a ceiling 7.

Liquid can be introduced into the container via a dip tube 9. Due to the supply of the liquid through the dip tube 9, an unacceptably large amount of turbulence can be prevented from occurring in the liquid during the introduction of the liquid into the container 1. A further reduction of turbulence during the decanting of the liquid into the container 1 can be achieved in that a baffle plate 11 is positioned below the dip tube 9. The liquid flowing in through the dip tube 9 flows onto the baffle plate 11, and is thus deflected and distributed, so that in accordance with the design of the baffle plate 11 or the angle at which the baffle plate 11 is arranged below the dip tube 9, targeted flow widening can be set. A further advantage of the baffle plate 11 is that the inflowing liquid does not strike the container bottom 3 directly and entrain and swirl up solids possibly accumulated there in this way, so that they are distributed in the liquid. The container is designed in the embodiment shown here in this case so that there is always enough liquid in the container 1 that the dip tube 9 is still immersed in the liquid when the container 1 is emptied.

The removal of the liquid is performed, for example, via a submersible pump 13. The submersible pump 13 is also submersed in the liquid in this case. Liquid can be removed from the container via the submersible pump 13 until the intake connecting piece 15 of the submersible pump 13 is no longer submersed in the liquid. The minimal fill level of the liquid in the container 1 thus results due to the location of the intake connecting piece.

The liquid sucked in by the submersible pump 13 flows out of the container 1 through a flow pipe 17. The drive of the submersible pump 13 is performed using a pump shaft 19, which is guided through the cover 7 of the container 1. For protection against entering liquid, the pump shaft 19 is guided in a pipe 21. Since in particular in the case of long submersible pumps, i.e., in the case of great height of the container 1 and correspondingly long flow pipe 17 and pump shaft 19, the flow pipe 17 and the pump shaft 19 are segmented, flow pipe 17 and pump shaft 19 are preferably guided in an envelope pipe 21. The envelope pipe 21 prevents uncontrolled gas exchange between the lower side and the upper side of the floating roof. It is preferable for the envelope pipe to be sealed against the gas phase above the floating roof, while it is open at the lower end. This prevents gas loaded with a high concentration of nitrogen oxides from penetrating into the gas compartment above the floating roof. The envelope pipe preferably has a sufficiently large diameter that the dip tube can be pulled through the envelope pipe, for example, for maintenance purposes.

In the embodiment shown here, a distributor 25 is located below the submersible pump 13. It can be embodied in the form of a perforated floor, for example. The distributor is located in this case at the position of the lowest liquid level in the container 1. The distributor 25 is used to dampen turbulence which can arise due to the inflow of the liquid, so that the liquid remains calm above the distributor 25 and no waves arise on the surface, due to which the floating roof 29 can begin to move. If, as shown here, a distributor 25 is provided, the possibility exists of, for example, guiding the dip tube 9 through a feedthrough 27 in the distributor 25 and fixing it on the distributor 25. In addition, the envelope pipe 21 of the submersible pump 13 can also be fixed on the distributor 25. The fixing of dip tube 9 and submersible pump 13 prevents them from beginning to oscillate and thus being able to cause damage to installations or to the container 1.

According to the invention, a floating roof 29 is accommodated in the container 1. The floating roof 29 floats in this case on the surface 31 of the liquid in the container. For this purpose, floats 33 are formed on the floating roof 29, which float on the liquid and support the floating roof 29. In the embodiment shown here, the entire floating roof 29 is not in contact with the surface 31 of the liquid, but rather only the floats 33. However, it is alternatively also possible to form the entire floating roof 29 in the form of floats, so that the entire floating roof 29 floats on the surface 31 of the liquid. In particular in the case of use as a hot tank of a solar-thermal power plant, it is preferable if the floating roof is embodied as thermally insulating. For this purpose it is possible, for example, to design the floating roof 29 as a hollow body and to fill it with an insulation material. Alternatively, the possibility also exists of manufacturing the floating roof 29 entirely from the insulation′ material. In particular steel-plate-clad ceramics having gas inclusion, for example, foamed ceramic or foamed glass, which are temperature-stable and pressure-stable and enable very thin cladding plates to be used, are suitable as insulation materials. Alternatively, it is also possible, for example, to use typical inorganic fibre mats for thermal insulation, but then occurring external pressures must be absorbed by an envelope which is embodied as sufficiently stable.

Feedthroughs for installations are formed in the floating roof in the embodiment shown here. The dip tube 9 is guided through a first feedthrough 35 and the submersible pump 13 is guided through a second feedthrough 37. In this case, the second feedthrough 37 is made sufficiently large that the pump head of the submersible pump 13 can be inserted through the feedthrough into the container.

In order that no gas can escape from the liquid through the floating roof 29, the feedthroughs 35, 37 are preferably provided with a movable sealing plate 39. The movable sealing plate 39 is designed in this case so that it can both rise and fall vertically with the floating roof 29 and a horizontal movement is additionally possible, to prevent excessively large force action on the installations, by which damage can be induced, in the event of pipes of the installations which do not extend completely vertically, for example, dip tube 9 and envelope pipe 23.

To prevent tilting of the floating roof 29 when the floating roof 29 rises and falls, a guide 40 is preferably provided, along which the floating roof 29 is guided. For example, a guide rod can be attached on the container wall 5 as the guide 40 and the floating roof 29 encloses the guide rod so that the floating roof 29 is moved along the guide rod. Alternatively, it is also possible to provide guide rods in the container interior, which are guided through corresponding feedthroughs in the floating roof 29. In addition, the installations, for example, the dip tube 9 or the envelope pipe 23 of the submersible pump 13, can also be used as the guide.

A gas compartment 41 is above the floating roof 29, between floating roof 29 and cover 7 of the container. To prevent the gas in the gas compartment from being compressed when the floating roof 29 rises, a gas outlet 43 is provided in the cover.

If the container 1 is part of a two-tank system, for example, as is used in solar-thermal power plants, in which the colder liquid is stored in a first container and the warmer liquid is stored in a second container, so that in each case one container is emptied and the other is filled accordingly, it is preferable if the containers are connected to one another via the gas outlet 43 in the cover, so that in each case the gas can flow out of the container which is emptied into the container which is filled. In the case of thermal insulation of the floating roof 29, it is possible in this case that the gas phases in the first container and in the second container have essentially equal temperature and therefore, at equal pressure, also equal specific volume.

FIG. 2 shows a detail of the floating roof 29.

The floating roof 29 is guided using a sliding seal 45 on the container wall. The compartment below the floating roof 29 is sealed off in relation to the gas compartment 41 using the sliding seal 45, so that no decomposition gas arising from the liquid can escape into the gas compartment 41. Furthermore, this also prevents gaseous and liquid contaminants in particular from being able to reach the liquid from the gas compartment 41.

To improve the leak-tightness, it is advantageous if a sealing lip 47 is additionally located above the sliding seal. The sealing lip 47 is guided in this case along the container wall 5 and has an additional sealing action.

Below the sliding seal 45, ribs 49 are formed on the floats 33. The ribs 49 are spaced apart from one another, so that a gas compartment 51 is formed in each case between the ribs 49. The ribs 49 can be used as an additional seal. Furthermore, in particular the gas compartment 51 acts as additional insulation, so that the temperature in the region of the sliding seal 45 is lower than directly above the liquid. In this way, the sliding seal 45 is protected from excessively high temperatures and possible damage as a result of the high temperatures. In particular, it is also possible in this way to use sealing materials which would be damaged at the high temperatures of the liquid.

To furthermore prevent liquid creeping upward from the container from coming into contact with the sliding seal 45, it is advantageous to attach a drip edge 53 above the sliding seal 45. Liquid creeping upward drips off on the drip edge 53 and falls back downward into the liquid.

In contrast to the embodiment shown in FIG. 1 having unfilled floats 33, the floats are filled with a thermally insulating material 55 in the embodiment shown in FIG. 2. This prevents the floats from acting as thermal bridges and dissipating heat from the liquid to the gas compartment 41 above the floating roof 29.

FIG. 3 shows a solar-thermal power plant in which at least one container 1 having floating roof 29 is used.

In a solar-thermal power plant having a first container 57 for storing a colder liquid and a second container 59 for storing a hotter liquid, at least the second container 59 is equipped with a floating roof 29. In the embodiment shown here, the floating roof 29 is constructed from multiple segments 61, which are connected to one another so they are movable. The segments 61 are each equipped with floats in this case, so that each segment floats per se on the surface of the liquid. The liquid which is stored in the first container 57 and in the second container 59 is used as a heat carrier medium and is typically a molten salt. Salts which are used for the molten salt are in particular nitrates and nitrites of the alkali metals and alkaline earth metals and also arbitrary mixtures thereof. A typically used salt is a mixture of sodium nitrate and sodium nitrite in the weight ratio of 60:40.

In operation of the solar-thermal power plant, at times having incident solar radiation, the liquid is removed from the first container 57 and conducted through a solar field 63. The solar field 63 has receivers 65, in which the liquid is heated by incident solar energy. The liquid thus heated is introduced into the second container 59. In this case, the liquid volume decreases in the first container 57, whereby the gas compartment enlarges. At the same time, the liquid volume increases in the second container 59, so that the gas compartment 41 in the second container 59 shrinks. In this case, the gas from the gas compartment of the second container 59 is introduced via a gas pendulum line 67 into the first container 57. Excess gas, which can arise, for example, due to outgassing of gases dissolved in the liquid, which can enter the gas phase, for example, if the first container 57 is not equipped with a floating roof, can be removed via a gas outlet 69.

To generate power, the hot liquid from the second container 59 is supplied to a first heat exchanger 71 of a steam cycle 73. In the first heat exchanger 71, the water is vaporized and superheated by a heat transfer from the hot liquid to the water cycle. The superheated steam thus generated drives a steam turbine 75, which in turn drives a generator 77 to generate power. The superheated steam is relaxed in this case in the steam turbine 75.

The steam flowing out of the steam turbine 75 is condensed in a second heat exchanger 79, wherein the heat from the water of the steam cycle 73 is transferred to a cooling cycle 81. The cycle 81 is typically also operated using water, wherein the water of the cooling cycle 81 is cooled down in a cooling tower 83.

After the condensation, the water of the steam cycle 73 is compressed using a pump back to the pressure which is required to drive the steam turbine 75, before the water again flows into the first heat exchanger 71 for vaporization and super heating.

For example, parabolic troughs or Fresnel receivers can be used as the receivers 65 in the solar field 63. Alternatively, it is also possible to use a central receiver of a tower power plant instead of the solar field 63, wherein the liquid is then heated in the tower.

List of reference numerals 1 container 3 container bottom 5 container wall 7 cover 9 dip tube 11 baffle plate 13 submersible pump 15 intake connecting piece 17 flow pipe 19 pump shaft 21 pipe 23 envelope pipe 25 distributor 27 feedthrough 29 floating roof 31 surface of the liquid 33 float 35 first feedthrough 37 second feedthrough 39 movable sealing plate 40 guide 41 gas compartment 43 gas outlet 45 sliding seal 47 sealing lip 49 rib 51 gas compartment 53 drip edge 55 thermally insulating material 57 first container 59 second container 61 segment 63 solar field 65 receiver 67 gas pendulum line 69 gas outlet 71 first heat exchanger 73 steam cycle 75 steam turbine 77 generator 79 second heat exchanger 81 cooling cycle 83 cooling tower 

1-13. (canceled)
 14. A container for storing a liquid, which tends to decompose into gaseous decomposition components in the case of the conditions prevailing in the container (1) and in the case of which a chemical reaction in equilibrium results between gaseous decomposition components and liquid, wherein a floating roof (29) is accommodated in the container (1), wherein the floating roof (29) comprises floats (33), using which the floating roof (29) floats on the liquid, and wherein the floating roof (29) is guided using a sliding seal (45) in the container (1), characterized in that the sliding seal (45) is thermally insulated from the liquid stored in the container (1).
 15. The container of claim 14, wherein the floating roof (29) is constructed from at least two segments (61), wherein the segments (61) are connected to one another in a movable manner.
 16. The container of claim 14, wherein the floating roof (29) has at least one chamber, which contains thermally insulating material.
 17. The container of claim 14, wherein feedthroughs (35, 37) are formed in the floating roof (29).
 18. The container of claim 17, wherein the feedthroughs (35, 37) are sealed using a movable sealing plate (39).
 19. The container of claim 14, wherein the sliding seal (45) has protective units (53) against liquid creeping upward.
 20. The container of claim 14, wherein the floating roof (29) is guided on at least one guide in the container.
 21. The container of claim 14, wherein the floating roof (29) has units to compensate for thermal expansion.
 22. The container of claim 14, wherein the liquid is a molten salt.
 23. The container of claim 22, wherein the molten salt is a mixture of nitrite and nitrate salts.
 24. A device for storing heat, comprising a first container (57) for storing a colder liquid and a second container (59) for storing a hotter liquid, wherein the containers (57, 59) are connected to one another, so that the colder liquid flows out of the first container (57), after absorbing heat, into the second container (59) and flows out of the second container (59), after emitting heat, into the first container (57), wherein at least the second container (59) is a container according to claim
 14. 25. The device of claim 24, wherein a gas compartment (41) is formed in the first container (57) and in the second container (59) above the floating roof (39), and the gas compartments (41) of the first (57) and the second (59) container are connected to one another via a connecting line (67).
 26. A solar-thermal power plant comprising the container of claim
 14. 27. A solar-thermal power plant comprising the device of claim
 25. 